Adding another email issue

Adding another email issue

I use Mozilla Thunderbird for my emailing.  My accounts are hosting at Zoho.  Anyhow, when I signed up for zoho, i used my personal email.  When I try to send an email from the lead screen, it will allow me to do so but it is from my personal.  I clicked add another email, but when I put my business email in, it says the email already exists...thoughts?